fix(material/expansion): move unthemable tokens to theme mixin

Though these tokens are not currently affected by the theme, in the
future they will be affected by the design system used for theming (M2
or M3)

BREAKING CHANGE:
There are new styles emitted by mat.expansion-theme that are not
emitted by any of: mat.expansion-color, mat.expansion-typography,
mat.expansion-density. If you rely on the partial mixins only and don't
call mat.expansion-theme, you can add mat.expansion-base to get the
missing styles.

Author: mmalerba

src/material/expansion/_expansion-theme.scss

@@ -6,7 +6,10 @@
 @use '../core/tokens/m2/mat/expansion' as tokens-mat-expansion;
 
 @mixin base($theme) {
-  // TODO(mmalerba): Move expansion panel base tokens here
+  @include sass-utils.current-selector-or-root() {
+    @include token-utils.create-token-values(
+        tokens-mat-expansion.$prefix, tokens-mat-expansion.get-unthemable-tokens());
+  }
 }
 
 @mixin color($theme) {

src/material/expansion/expansion-panel.scss

@@ -6,8 +6,6 @@
 @use '../core/style/elevation';
 
 .mat-expansion-panel {
-  @include token-utils.create-token-values(
-    tokens-mat-expansion.$prefix, tokens-mat-expansion.get-unthemable-tokens());
   @include elevation.overridable-elevation(2);
   box-sizing: content-box;
   display: block;